Charge trapping and photoadsorption of O2 on dehydroxylated TiO2 nanocrystals - An electron paramagnetic resonance study

The interaction of photogenerated charges with molecular oxygen was investigated on TiO2 nonocrystals by means of paramagnetic resonance (EPR) spectroscopy. Compared to photoactivation experiments in Vacuum at p < 10(-6) mbar and T = 740 K, the presence of O-2 enhances the concentration of persistently trapped electron and hole centres-by a factor of ten-due to the formation of adsorbed O-2(-) species. The photoadsorption of oxygen was also tracked quantitatively by pressure measurements, and the number of trapped charges, hole centres and 0,was found to correspond to ten electron-hole pairs per TiO2 nanocrystal. Conversely, in experiments at p < 10(-6) mbar with one trapped electron-hole pair per particle, charge separation is not persistent and completely reversible with respect to temperature. Heating to 298 K causes the total annihilation of photogenerated and trapped charges